1. Field of the Invention
This invention relates to pneumatic tires, and more particularly to an improvement of a tread pattern in such a pneumatic tire.
2. Related Art Statement
Heretofore, there have been proposed various methods for reducing noise generated from a pneumatic tire. For example, it is known to change the pitch of the tread pattern in the circumferential direction of the tire. In this case, plural circumferential ribs arranged in the tread of the tire are constructed by arranging plural pitch groups, for example, three pitch groups A, B and C at a regular order in each of the circumferential ribs in the circumferential direction thereof. Each of the pitch groups is comprised of at least one pitch element such as block having the same pitch length. When the pitch of the tread pattern is changed in the circumferential direction as mentioned above, frequency components of peak orders in ground striking sounds are dispersed to reduce the pattern noise to a significant extent and render it into white noise.
In such a pneumatic tire, however, the different pitch elements repeatedly strike on ground at a regular order during the running, so that the magnitude of output to the tire changes with the lapse of time. Furthermore, when the frequency component order of the ground striking sound generated from one kind of these pitch elements is coincident with the frequency of vibration of normal mode in the tire, such a frequency component is resonantly amplified. Since such amplification is repeated each striking of the above pitch element on ground, amplitude modulation (beats) is produced in the noise. Furthermore, the noise generated from this pneumatic tire is a mixture of various noises, and is offensive to the ear and is uncomfortable.